This invention pertains to a method of measuring heat influx of cryogenic systems, particularly those systems containing a cryogen flow.
Cryogenic transfer systems usually consist of an inner conductor portion maintained at cryogenic temperatures, surrounded by one or more insulated layers that thermally isolate the cryogenic conductor portion from the environment surrounding the system. The insulating layers may include circulating liquid or gaseous cryogen systems, or may include static systems, such as a blanket of aluminized mylar. Despite the careful design of such insulation systems, heat influx into the cryogenic portions is inevitably experienced during operation of the cryogenic system. If the heat influx exceeds a critical level, cryogenic temperatures necessary for successful operation the system cannot be maintained. It therefore becomes necessary to accurately measure the heat influx of a cryogenic system.
One such cryogenic system is commonly termed a "bayonet" type transfer line or interconnection between a cryogenic dewar, or source of cryogen, and a cryogenic apparatus which receives the cryogen. Such bayonet transfer lines usually assume an inverted U-shaped configuration. Prior testing methods of such bayonet transfer lines require the transfer lines to be disconnected and inverted, to present a generally U-shaped configuration. Liquid nitrogen, a relatively inexpensive cryogen, is then poured into a first end of the transfer line to fill as much of the transfer line as possible. One end of the transfer line is then sealed, and a gas flow meter is installed at the other end. The gas flow meter measures the liquid nitrogen boil-off which results from any heat influx into the transfer line. The mass flow rate of the liquid nitrogen boil-off is then empirically related to a heat influx rate.
This method is used for transfer lines carrying both nitrogen as well as helium cryogen, since the amount of boil-off of liquid helium would be prohibitively expensive, were that cryogen used during the measurement. Several problems are encountered when nitrogen cryogen is used to measure the heat influx of helium transfer lines. The heat flux into a helium transfer line has components of internal gas conduction, thermal radiation, and conduction experience at the end of the transfer line. There is no known theory of thermodynamics that will predict the operation of a helium system, given the parameters of operation of the same system, but with a nitrogen cryogen.
Another difficulty encountered with the aforementioned test method, is that the transfer line must be inverted so as to provide a receptacle for the liquid nitrogen poured therein. It is particularly difficult using the aforementioned test method to accurately measure the heat influx of a bayonet-type transfer line having one or more vertical bayonet connections. In a vertical bayonet connection, a male bayonet component is oriented to point in a downward position, with the transfer line inlet/outlet at the bottom-most portion of the bayonet. When installed in a transfer system, the male bayonet is received within an upwardly extending female bayonet component, having an opening at its uppermost portion. A column of cryogen gas extends the entire length of the bayonet connection, acting as an effective thermal insulator between the free end of the male bayonet and the surrounding environment. When the transfer line is disconnected and inverted, the gas column insulating the male bayonet is destroyed. It then becomes impossible using the aforementioned test method to measure the heat influx of the male bayonet portion of the transfer line assembly. This restriction is undesirable since the heat influx of the male bayonet very often represents the majority of the total transfer line heat influx. A more meaningful evaluation of a transfer line's thermal stability could be realized if the transfer line were tested in its normal operating position, with its test connections made as similar as possible to its operating connections.
It is therefore an object of the present invention to provide a heat influx measurement method wherein the cryogenic transfer line is maintained in its normal operating position with test connections similar to the transfer lines operating connections.
It is another object of the present invention to provide a heat influx test method utilizing the operating cryogen, maintaining that cryogen at the actual operating temperature.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.